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Re: [FAQ] XFS speculative preallocation

To: Arkadiusz MiÅkiewicz <arekm@xxxxxxxx>
Subject: Re: [FAQ] XFS speculative preallocation
From: Brian Foster <bfoster@xxxxxxxxxx>
Date: Fri, 21 Mar 2014 14:02:41 -0400
Cc: xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <201403211809.03683.arekm@xxxxxxxx>
References: <20140321162920.GA3087@xxxxxxxxxxxxxx> <201403211809.03683.arekm@xxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Fri, Mar 21, 2014 at 06:09:03PM +0100, Arkadiusz MiÅkiewicz wrote:
> On Friday 21 of March 2014, Brian Foster wrote:
> > Hi all,
> > 
> > Eric had suggested we add an FAQ entry for speculative preallocation
> > since it seems to be a common question, so I offered to write something
> > up. I started with a single entry but split it into a couple Q's when it
> > turned into TL;DR fodder. ;)
> > 
> > The text is embedded below for review. Thoughts on the questions or
> > content is appreciated. Also, once folks are Ok with this... how does
> > one gain edit access to the wiki?
> 
> More questions or topics that can be converted to questions from me:
> 
> 1) Before preallocation kernel did things differently. AFAIK it wasn't the 
> same as allocsize=64k, was it? Is there a way to get old behaviour or 
> something similar to old behaviour?
> 

Going from the commit log that introduced speculative preallocation, it
appears that the behavior was effectively allocsize=64k. For reference:

        055388a3 xfs: dynamic speculative EOF preallocation

> 2) Is there a way to see which file got some preallocation and how big that 
> preallocation is? Scenario - something ate free space due to preallocation 
> and 
> from admin point of view it would be usefull to know which app did that and 
> how many MB was due to preallocation (vs real, written data).
> 

The common scenario is when du/stat reports a larger block usage than
file size, so the question of how much extra space is allocated is just
the difference between the two. I suppose we could include a simple
example of that in the first Q.

This isn't necessarily true in the case of sparse files. xfs_bmap prints
the extent information for a file, so it should be possible to determine
how much post-EOF space exists from looking at the extent that covers
EOF. That said, this strikes me as more "user guide" material than FAQ.

> > Linux 3.8 (and later) includes a scanner to perform background trimming
> > of files with lingering post-EOF preallocations. The scanner bypasses
> > files that have been recently 
> 
> What time is "recently" ? Is "modified" equal to "file data modified" or 
> "file 
> data or metadata modified" ?
> 

I originally had something like "files that have not been modified since
last flushed to disk," which is the heuristic as I understand it. That
seemed too verbose and technical for FAQ. I could replace "recently
modified" with "... bypasses files that are dirty ..." if that is more
useful..?

> > modified to not interfere with ongoing
> > writes.
> 
> In case of some app that constantly writes to files (apache web server 
> writting to its logs for example) that background trimming will never do 
> anything for these files, right?
> 

Most likely true. Though by the same logic, those files will eventually
use the preallocated space.

> > A 5 minute scan interval is used by default and can be adjusted
> > via the following file (value in seconds):
> > 
> >     /proc/sys/fs/xfs/speculative_prealloc_lifetime
> > 
> > Although speculative preallocation can lead to reports of excess space
> > usage, the preallocated space is not permanent unless explicitly made so
> > via fallocate or a similar interface. Preallocated space can also be
> > encoded permanently in situations where file size is extended beyond a
> > range of post-EOF blocks (i.e., via truncate). Otherwise, preallocated
> > blocks are reclaimed on file close, inode reclaim, unmount or in the
> > background once file write activity subsides.
> 
> So there is no mechanism that would shirnk preallocations in case when free 
> space is (almost or) gone on a fs? Case: apache causes xfs to preallocate 
> several GB for its /var/..../{access,error}_log (common problem here) and 
> then 
> free space ends on that fs causing problems for every app that writes to /var.
> 

I noted in the second answer that the preallocation is throttled as we
near allocation limits such as no free space or quota. I think that
should cover most cases. I still have some code lying around somewhere
that forces a scan and retry in EDQUOT scenarios though. I should dust
that off...

Thanks for the reviews!

Brian

> Thanks!
> 
> -- 
> Arkadiusz MiÅkiewicz, arekm / maven.pl
> 
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